My first suspicion at this point is that a past owner, not knowing the VIN was stamped above the oil filter, had the VIN stamped into the pad... I know when I bought my car, and didn't see the VIN stamped (as other cars I'd owned) on the pad, I *wondered* should I stamp it in myself.. I Now am very glad that I didn't do anything so stupid..
Of course it's also possible that someone 'new' or forgetful' in the stamping station of the plant stamped it in the 'old' place, and then someone informed him, or he remembered, to stamp it in the 'new' location as well.